The present invention relates to a process for producing low-salt silica sol dispersions in low-boiling alcohols and to the use of the organosols obtained by this process for the production of coatings on metallic and nonmetallic substrates.
Silica sol dispersions in organic solvents have long been known (see R. K Iler, The Chemistry of Silica, pages 412-415, John Wiley and Sons, New York, 1979).
Silica sols dispersed in an organic dispersing medium which has a higher boiling point than water at atmospheric pressure can be produced in a simple manner from aqueous sols. An aqueous silica sol dispersion is treated with the higher boiling organic solvent and the water is removed by distillation. Processes which employ this principle are described in detail in the literature: GB-A-965 123, EP-A 372 124, U.S. Pat. No. 2,921,913, U.S. Pat. No. 2,974,105, U.S. Pat. No. 3,004,92, U.S. Pat. No. 3,629,139, U.S. Pat. No. 3,634,288, U.S. Pat. No. 3,336,235, U.S. Pat. No. 3,699,049.
Silica sols in organic dispersing media which have a lower boiling point than water at atmospheric pressure are far more difficult to obtain. Laborious, multi-stage processes are firstly required, starting from expensive raw materials. DE-A 1 935 752, GB-A 1 136 016 and GB-A 1 246 032 describe the electrolytic preparation of an organosol such as this. A ferrosilicon anode is used as the source of silicon. The organic dispersing medium consists of a monohydric alcohol containing 1 to 3 C atoms, small proportions of water and a specified conducting electrolyte. The process is laborious and energy-intensive, and is also disadvantageous economically due to the use of expensive raw materials. Low-salt silica sols (or even low-electrolyte silica sols) thus cannot be obtained in one process step.
However, low-salt silica sols in organic low-boiling media are particularly desirable for the purpose of coating surfaces, because corrosion and blistering effects caused by a salt content do not occur.
The ideal starting materials would be the aqueous silica sol dispersions which are readily obtainable in a simple and inexpensive manner and which are available commercially in large amounts. These constitute the primary product of the industrial production of silica sols and can readily be produced in different concentrations and different sizes (diameters of about 5 to 100 nm). Other silica sots, e.g. those which have been made acidic or which have been surface-modified or subjected to ion exchange, or the salt content of which has been altered, are obtained therefrom by subsequent treatment.
The transition from the ferrosilicon anode to these aqueous silica sols as the starting material constituted a significant step forward in the technology, whereby low-salt silica sols were accessible at the same time. U.S. Pat. No. 2,801,185 and U.S. Pat. No. 3,351,561 describe the production of a silica sol in a monohydric alcohol which is miscible with water and which has a boiling point &gt;50.degree. C. The starting material for this process is an aqueous silica sol which is salt-free (U.S. Pat. No. 3,351,561) or which has a low cation content (U.S. Pat. No. 2,801,185). The silica sols to be used have to be produced in an upstream process step by treating the alkaline silica sots, which are the primary products, with ion exchangers. Solvent exchange is then effected by distillation. In silica sols which are obtained by this process, a large number of the OH groups on the particle surface are esterified by the alcohol used. This process for producing silica sols in organic solvents is a two-stage process when readily available, commercially customary, low-salt silica sols are used as the starting materials, and provides a product in which a large number of the OH groups on the particle surface are esterified by the alcohol used. An esterification such as this is a disadvantage for the application of the product for coating purposes, since reactive points of bonding to the surfaces to be coated are thereby removed. Co-condensation with silanes for example--which is advantageous for the application of the product for coating purposes--is also repressed due to the lack of free OH groups on the particle surface. The specific hydroxylated surface of the particles is less than 10 m.sup.2 /g for products according to U.S. Pat. No. 2,801,185.
U.S. Pat. No. 3,855,145 describes the production of a low-salt silica sol, in n-propanol or in glycols for example, without esterification of the surface OH groups by the solvent taking place. In this process, an aqueous silica sol is treated first with a cation exchange resin and then with an anion exchange resin. The ion exchange resins are employed in a fixed bed process. So as to ensure that the electrolyte is removed as extensively as possible, treatment with the ion exchange resins is repeated after a period of 12 to 24 hours. After deionisation, exchange of the solvent is effected by distillation in a subsequent, further process step. The temperature during distillation is less than 100.degree. C. The steps of deionisation and water removal are process steps which are spatially and chronologically separate from each other. In this connection, the deionisation step is associated with an additional consumption of time which is considerable. According to this patent specification, low-salt silica sols can only be obtained by an expensive, multi-stage process.
A process is described in EP-A 607 710 for the production of silica sol dispersions in alcohols containing 1 to 3 C atoms, by pervaporation of a silica sol dispersion dissolved in water and alcohol, with removal of the water down to less than 1% by weight. This process only results in a low-salt silica sol dispersion in alcohol by employing a plurality of process steps.